Computer animation with particle system technology addresses a growing need to simulate complex systems for predicting real world performance. A complex system is modeled as a collection of a large number of constituents called particles. Particles interact among themselves, with other particle sources, and with surrounding surfaces or obstacles. They can be subject to external natural forces such as gravity, winds, and collisions with other particles, and can also be coupled with a surrounding medium such as a fluid flow. The results can then be output as a three dimensional (3D) rendered visual to view the system simulation results.
Particle systems were first introduced in the early 1980's for the modeling and the visualization of natural fuzzy phenomena like clouds, water, gasses and fire. The principle consists of defining objects as clouds of elementary primitives, which are the particles, and assigning to each of them physical attributes that determine their temporal evolution in terms of physical laws.
Particles are created in a system by a particle source or emitter. An emitter defines each particle with such characteristics as a position in space, dimension, size and geometry, rate of emission as a function of time, and direction of emission, which is typically given by a vector, a local normal to a surface, or a given trajectory.
In order to model objects with a group of particles, it is desirable to have the group of particles conform to an underlying structure or path. The particles are defined to interact with the path and each other such that a large object may be modeled over time. By defining a 3D path an object may be formed having the underlying structure of the path when the emitted particles are constrained along the direction of the path.
One well-known way of constraining a particle along a path is through the use of metaballs. A metaball is a density-based, isopotential surface, which is a field of matter that comprises a solid core with a visible surface and a semi-solid area of influence that decreases by the distance. When areas of influence of two or more metaballs overlap, their densities fuse together forming a new and more complex surface. Metaballs are ideally suited to modeling of smooth organic forms such as the human body. The surface of a metaball defines a boundary along the path which the object particles must not cross. In other words, a particle created by an emitter has its direction and velocity constrained by the boundary created by the metaballs along the path of the intended object.
Because metaballs define a boundary across which particles may not cross, it may be difficult to use them to model naturally fuzzy objects with no easily discernible boundaries, such as fire. Therefore, there is a need for a way to allow a particle to follow a path which does not constrain the particle by the use of boundaries.